Lets look at 3 ways that God's Word allows for the possibility of ultimate salvation for those have never heard the Gospel:
(1) In Romans Paul addresses the question of the redemptive possibilities for those who have not been exposed to the Law or the Bible and therefore have never heard the Gospel: "When Gentiles who do not have the Law do instinctively what the Law requires,...they show that what the Law requires is written on their hearts, to which their own conscience also bears witness (2:14-15)." These pagans can be saved simply by "patiently doing good" and by seeking "for glory, honor, and immortality:" "To those who by patiently doing good seek for glory, honor, and immortality
He will give eternal life (2:7)." Why? Because "sin is not imputed where there is no Law (5:13)." Notice carefully that these pagans "seek for glory, honor, and immortality," but do not know enough to seek for salvation by grace through faith in Christ's finished atonement. Thus, Paul can say this about those who have never heard the Gospel: "God has overlooked the times of human ignorance (Acts 17:30)." These are now required to repent when and only when they hear and grasp the Gospel.
(2) But even the wicked dead in Hades can receive a 2nd chance to get saved:
"He (Christ) or this went and made a proclamation to
the spirits in prison, who in former times did not obey (
1 Peter 3:19-20)."
The phrase "a proclamation to the spirits is prison" is picked up in 4:6 by the phrase, ""the Gospel was proclaimed even to the dead" and it is now made clear that they are allowed to respond to the Gospel and be saved (= "live in the spirit"):
"For this reason,
the Gospel was proclaimed even to the dead, so that, though they had been judged in the flesh, as everyone is judged, they might live in the spirit as God does (4:6)."
(3) Proxy baptism for the unsaved dead can be part of the process by which God becomes "everything to everyone" and by which they too can ultimately be saved:
...so that God may be everything to everyone (or: all in all").
Otherwise, what will those people do who receive baptism in behalf of the dead? If the dead are not baptized at all, why are they baptized on their behalf (
1 Corinthians 15:28-29)?"
The word "otherwise" is key: in other words, if God is not ultimately going to "be everything to everyone," what is the point of proxy baptism for the sinful dead?"